The present invention generally relates to ink printing technology, and more particularly to an ink composition which is formulated to avoid outgassing (e.g. internal gas bubble generation) during delivery by thermal printing systems.
Substantial developments have been made in the field of electronic printing technology. A wide variety of highly efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially important in this regard. Printing systems using thermal inkjet technology basically involve a cartridge unit which includes at least one ink reservoir chamber in fluid communication with a substrate having a plurality of resistors thereon. Selective activation of the resistors causes thermal excitation of the ink and expulsion from the ink cartridge. The ink composition is specifically expelled through one or more tiny openings in a metallic member known as an "orifice plate".
Thermal inkjet cartridges may contain either a single ink composition (e.g. black) or a plurality of different colored ink materials each stored within a separate compartment or chamber. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al., U.S. Pat. No. 4,794,409 to Cowger et al.; U.S. Pat. No. 4,509,062 to Low et al.; U.S, Pat. No. 4,929,969 to Morris; U.S. Pat. No. 4,771,295 to Baker et al.; and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein by reference.
To obtain a high level of print quality using a thermal inkjet system or other printing apparatus, the ink composition must be carefully formulated. Likewise, reliability of the ink cartridge during operation is of primary consequence. The ink cartridge and ink composition within the cartridge should be designed so that the ink may be delivered without interruption over a sustained period of time. Many internal factors may cause the interruption of ink delivery during cartridge operation. For example, it is important to avoid the presence of extraneous solid materials within the ink which may block various components inside the cartridge unit, including the openings in the orifice plate. Careful processing and strict quality controls during the ink manufacturing process will control this problem. It is also necessary to prevent the growth of microorganisms (e.g. bacteria) within the ink composition. The presence of microorganisms can adversely affect print quality and ink cartridge operation. A number of anti-microbial agents may be used to avoid microorganism-related problems.
However, one problem of particular concern in thermal printing systems (e.g. thermal inkjet printers) is a condition known as "outgassing". The term "outgassing" basically involves the formation of gas bubbles directly within the ink composition during temperature increases which are normally encountered in thermal printing systems. The gas bubbles are comprised of gaseous materials which were previously dissolved in the ink compositions as discussed below. Typically, ink materials which are delivered using thermal inkjet technology are heated to an average temperature of about 25-80.degree. C., depending on the printing system being used. At temperatures within this range, the solubility of any air or other gases dissolved within the ink composition will decrease substantially. This condition (which is especially true in connection with water-based inks designed for plain-paper printing) causes supersaturation of the ink composition with the dissolved gases. In turn, the equilibrium kinetics associated with supersaturation will cause gas bubbles to form in the ink.
Many factors will determine whether dissolved gases are present within a particular ink composition, as well as the level of gas saturation in the ink. The initial dissolution of air or other gases (e.g. CO.sub.2, He, Ne, Ar, N.sub.2, O.sub.2) within ink compositions typically occurs during the ink manufacturing process and is difficult to avoid. Gases may become dissolved in a particular ink composition due to agitation of the ink, chemical interactions between ink components which generate various gaseous by-products, air diffusion through specific components of the printing system (e.g. plastic cartridge housings, ink nozzles, etc.), and other factors. However, when dissolved gases are present in an ink composition, tests have shown that the outgassing/bubble generation rate will typically double when the ink temperature is raised from about 50.degree. C. to 65.degree. C. Further general information involving the relationship between increased fluid temperatures and diminished gas solubility is discussed in Gjaldbaek, J. C., et al., "The Solubility of Nitrogen, Argon and Ethane in Alcohols and Water", Acta Chemica Scandinavica, 12(5):1015-1023 (1958), and Lannung A., "The Solubilities of Helium, Neon and Argon in Water and Some Organic Solvents", J. Am. Chem. Soc., 52:68-80 (1930) which are incorporated herein by reference. The correlation between temperature and the solubility of non-polar gases in non-polar liquids is expressed in accordance with the following general formula presented in Gjaldbaek et al.: EQU -log x.sub.2 =-log x.sup.i.sub.2 +0.4343(V.sub.2 /RT)(.differential..sub.1 -.differential..sub.2).sup.2 +log(V.sub.2 /V.sub.1)+0.4343(1-[V.sub.2 /V.sub.1 ])
[wherein:
x.sub.2 =solubility in mole fraction; PA1 x.sup.i.sub.2 =the "ideal" solubility; PA1 V.sub.1 =the molal volume of the solvent; PA1 V.sub.2 =the partial molal volume of the dissolved gas; PA1 .differential..sub.1 =the solubility parameter of the solvent; PA1 .differential..sub.2 =the solubility parameter of the gas; PA1 R=the gas constant; and PA1 T=the absolute temperature.]
Gas bubble formation within an ink composition can impair ink delivery by the blockage of various internal components inside the ink cartridge. For example, gas bubbles may block ink flow through a variety of components including an internal conduit known as the "standpipe" and an ink filtration screen which is typically positioned adjacent the printhead inside the cartridge. In addition, gas bubbles can form within and block small channels inside a typical thermal inkjet cartridge unit which lead to the printing resistors. While temperature increases are the primary cause of outgassing, the degree of bubble formation may be influenced by other factors including (1) the level of agitation (e.g. mixing) experienced by the ink composition inside the ink cartridge unit during use; and (2) the number of "seed sites" or "nucleation areas" which are present in the cartridge. The terms "seed site" and "nucleation area" are substantially equivalent, and basically involve physical locations/structures where gases are likely to come out of solution. Multiple seed sites inside a cartridge unit can substantially increase the rate of bubble formation. Typical seed sites in a thermal inkjet cartridge will include the filtration screens described above, as well as silicon wafer structures present inside the cartridge unit and various filler materials (e.g. carbon and/or glass fibers) within plastic components used in the cartridge. Gas bubbles, themselves, may also function as seed sites for the formation of additional and/or larger bubbles in the ink. However, as previously indicated, temperature increases are the primary cause of decreased gas solubility and outgassing. In a typical ink composition of the type normally used in thermal inkjet printing systems, as much as about 2-10 cc of gas (mostly air) per liter of ink may be dissolved in the ink. The specific amount of gas to be released will depend on numerous factors including the temperature conditions encountered during ink delivery, the internal configuration of the ink cartridge unit, and other factors determined by experimental investigation. However, many ink compositions currently in use can release as much as about 1-5 cc of gas per liter of ink over a 1-5 hour period when subjected to temperatures between about 25-80.degree. C.
As discussed above, outgassing and bubble formation can cause numerous difficulties including the premature termination of ink delivery. A need currently exists for an ink composition which avoids problems caused by outgassing in thermal printing systems. The present invention satisfies this need in a highly efficient manner and represents a significant advance in the art of thermal inkjet technology as discussed below. Specifically, a unique ink composition and printing method are provided which prevent bubble formation in an effective manner without the need for physical modifications to the printing hardware.